The invention relates in general to analog data acquisition system and more particularly to analog signal multiplexing systems interfaced with a digital system through an analog to digital converter.
Analog data acquisition systems are used in many industrial applications, such a process control, data logging, supervisory instrumentation, etc. A typical analog data acquisition system of the prior art is described in U.S. Pat. No. 3,806,916 issued Apr. 23, 1974 of same assignee as the assignee of the present application. Basically, such systems derive analog information from a plurality of remote locations and gather the analog data at a central location where it is converted into digital form for subsequent digital treatment by a suitable utilization device, such as a computer. Essential components of the data acquisition system are the multiplexer selecting one of the plurality of input analog signals from the remote locations and the A/D converter into which each particular input analog signal is inputted.
The A/D converter is used to convert the input analog signal into a digital representation thereof, generally in the form of a pulse count corresponding to the inputted analog signal. Various types of devices can be used for such purpose. Most typically used is the dual slope integration device in which the signal is integrated during a fixed time and is counted during a variable time. Whatever the type used, the inputted analog signal must not exceed a certain level which would be beyond the capability of the particular A/D converter. In order to accommodate the inputted analog signals to the capability of the converter, several methods have been used in the prior art. One is to group the signals in accordance with a common range of magnitude and to associate one A/D converter of designed capability with each such grouping. This is an expensive solution. Another solution is to use a variable gain amplifier which is adjusted for each measurement to the gain required for proper operation by the A/D converter. However, this second solution raises some problems. Each inputted signal must be identified as to channel and gain and the selection of the gain must be accordingly accomplished on the amplifier. When many points must so be monitored by the data acquisition system, it can be appreciated that while the use of a local Analog Multiplex Substation (AMS) reduces the length of the cabling from the computer, nevertheless, an excessive core memory is needed to store both the address of each channel and the gain setting required. This is very costly. Moreover, the analog transducer outputs may vary in amplitude over a wide range at the neighboring locations, and grouping of transducer both as to space and range may not be possible. In such instances the analog input signal must be sequenced without regard to the amplitude ranges. Digital control of the gain has thus become necessary from the central computer. Software programming may then be required to randomly access each channel at the appropriate time in the acquisition or control sequence.
An object of the present invention is to provide an improved data acquisition system for the reception of analog signals of different ranges of magnitude.
Another object of the present invention is to provide a data acquisition system in which the received analog signals are automatically converted into signals within the capability of the A/D converter used in the system.
A further object of the present invention is to provide digital treatment of analog data acquired and converted by an A/D converter in which the range of the acquired analog signal is more easily and less expensively identified.